Bear in mind the PCM54 is a mono DAC, and is putting out a multiplexed signal to save costs. The distortion you see/hear at TP3 might be normal, as it consists of both Upper and Lower alternating at a very fast rate.
I´ve had a similar problem with my D550 and the culprits were the solder joints on IC13 that had to be reworked.

I dont have the service notes here atm but testing the chips should be easy. Just follow the input and output lines and watch with a scope or a logic probe the pulse streams. Thats the digital data or 1s and 0s in electric form.
For instance that 74hc chip has four lines going into the resistor network. Theyre the higher 4 bits. If theres 4 pulse trains the chip is most probably ok.

I dont have the service notes here atm but testing the chips should be easy. Just follow the input and output lines and watch with a scope or a logic probe the pulse streams. Thats the digital data or 1s and 0s in electric form.
For instance that 74hc chip has four lines going into the resistor network. Theyre the higher 4 bits. If theres 4 pulse trains the chip is most probably ok.

What tests did you run to find tp3 returns a distorted signal?u

I am not that trained in electronics but I will try your tips.

On TP3 I connected a headphone and the output sound distortion was there. Same way the output jack.

Mooger5 wrote:Well, the synth has been in storage for the last 20 years. Did you install a fresh battery and do a reset already?
Have you checked the PSU voltages?
Looked for bad capacitors?

Those routines always come first. We just assume upon reading the OP the obvious stuff is already done.

Be sure to AC couple your phones with a capacitor, before the DC ruins them.

Good luck.

PSU voltages are good. Replaced all capacitors on the board yesterday.
All capacitors around the DAC were fine.
There was one faulty near the CPU and was the C50.
I am waiting for my new DAC to arrive to then test everything.